Billing query

My Mom is a pensioner and has a FNB credit Card since August/September of 2018. She has on numerous occasions advises that her debit day be the 25th of the month. Every single month, she is advised that she has to pay an interest if she does not make payment. Now for more than 9 months she has been doing to the branch in Mitchell's Plain, Cape Town and they keep telling her to phone the call centre which she does and just when she thinks the problem is sorted the next month has the same issue. Now I am getting fed up with *********** people that can't follow instructions and every time she goes in its the same story different person. And for a pensioner to deal with this *********** people is wrong.

I want this issue sorted out asap. Never have I had to deal with FNB at this level but its seems the only way I can get assistance.